Okay, here me out: I am designing an massive, kilometers-long at the absolute minimum, creature that lives in asteroid belts in the vacuum of space. It hosts an entire ecosystem of living things like macroscopic unicellular organisms inside its body. It eats asteroids, using enzymes to break them down for sustenance, and has skin that keeps its insides pressurized and safe from harmful UV radiation. Its body retains heat so that it doesn't freeze in the vacuum of space that it spends its whole life in, and it is capable of reproduction. In order to move, it orbits its sun, and ejects waste to propel itself, slow down, and turn. How would that organism be scientifically plausible?
Scientifically possible, though unlikely
On the question of biological possibility, space does pose some challenges: a vacuum, radiation, and wild temperature variations to name a few. Luckily, we are aware of extremophiles capable of surviving in many of these conditions. Radiotrophic fungus found in Chernoybl lives off radiation. Thermus aquaticus can survive up to 80°C/176°F (a feature we've much exploited). Tardigrades can survive exposure to space itself as well close to absolute zero. Simply put, if life evolved on Earth can handle many of the worst problems of space (though not necessarily thrive), an organism specifically evolved for space should be possible.
Such an organism would exist in a symbiotic relationship with the ecosystem inside it, providing protection from the vacuum in exchange for the ability to process the contents of its food (similar to the gut microbiome of humans). The creature would have to be sole primary source of energy to the system, similar to how plants/phytoplankton allows Earth's ecosystem to capitalize on solar power. Nutrients are fairly scarce in space, and supplying the energy for both a kilometers long creature and its internal microbiome is no easy task. Evolution does not favor the inefficient, so max utilization of available energy is expected. Therefore, it should evolve the ability to passively absorb the light and radiation energy going through space via photosynthesis and radiosynthesis, respectively.
That's not to say there is no use for eating asteroids. For starters, in order to propel itself around the it needs to acquire mass to do so. For Newtonian physics reasons. In addition, the materials gained from an asteroid will probably be necessary to preform many hyperspecfic biological functions, which we can offload to the microbiome inside.
A creature that large should require a similarly sized amount of energy. However, not all parts of an organism actually need to be "alive". Only about 1% of a tree is actually alive. Similarly, many animals have developed shells which are hard but not full of living cells that need to be supported. This is a simple way to keep the desired size while reducing the energy needs. Since you can just float your way around in microgravity, the drawbacks of large amounts of dead weight like that are relaxed. Such would also block and leaked radiation from reaching the core. I'm imagining most of the creature is a hard shell/skeleton and the living part is merely essential organs and systems connecting them.
Asteroids are fairly far apart, which gives a long lead time to intercept them. Without any natural born space predators, it moves at the slowest speed it can get away with, minimizing the muscle mass it needs to support. Move in this sense would mean angling its propulsion or adjusting its angular momentum, as Newton's first law handles the actual locomotion part. Another organ I can presume is highly advanced eyes to scope out any asteroid hundreds/thousands of kilometers down its trajectory.
The biggest issue with its existence is its evolution. I simply can't see how such a creature would evolve nor how all it's evolutionary ancestors would survive in space. Though perhaps if the universe really is infinitely large, then a panspermia like process might have just the creature, or one that can conveniently adapt to its new situation floating in in asteroid ejected from some cataclysm on a planet where life actually evolved which happens to get trapped in the orbit of a star with just the right type of asteroid belt is one of the infinite possibilities the universe has to manifest.
Hard to believe scientifically speaking
There is a bit of a misconception about asteroid belts: they are not that dense of elements. First of all they are not uniformly spread. Secondly you can find some information about the approximative average distance between two "big-enough-to-be-detected" asteroids: ~3 million kilometers (we are speaking about asteroids >1km large). You can find this information on the Physics Stack Exchange if you want the maths behind.
I get you don't have to focus only on the big rocks (let's say it has a really slow organism so he don't need to eat alot) but you are saying your living entity is moving by using the ejection of wastes and at the same time this monster is kilometers long so it has to use something big to counterweight that to move to the next asteroid (which supposely is ~3M km, and by guessing your monster can detect them). Orbiting the Sun does not give you deltaV so if it slow down it has to reaccelerate.
So hardly believable in my opinion if you want to back this idea with hard science.